The present invention relates to the field of internal combustion engines. More particularly, the present invention relates to the catalytic treatment of exhaust gases that are generated by internal combustion engines.
Internal combustion engines generate exhaust gases when they burn air and fuel. The mixture of air and fuel used in internal combustion engines is called the xe2x80x9cair/fuel mixture.xe2x80x9d The air/fuel mixture may be characterized in terms of an air/fuel ratio, which describes how many kilograms of air per kilogram of fuel are supplied to the internal combustion engine.
The normalized air/fuel ratio lambda (xcex) is often used to describe the ratio of the components of the composition of this air/fuel mixture. Lambda refers to the air/fuel ratio normalized to stoichiometric conditions. The air/fuel ratio for stoichiometric combustion for conventional engine fuels is 14.7; the normalized air/fuel ratio lambda at this point is 1. Air/fuel ratios less than 14.7, or normalized air/fuel ratios less than 1, are called rich and air/fuel ratios greater than 14.7, or normalized air/fuel ratios greater than 1, are called lean.
During combustion of the air/fuel mixture, the internal combustion engine will generate harmful substances, such as carbon monoxide, hydrocarbons and nitrogen oxides. If no storage effects for certain components of the exhaust gas are present in the internal combustion engine, then the normalized air/fuel ratio of the exhaust gas corresponds to the normalized air/fuel ratio of the air/fuel mixture supplied to the engine.
In order to achieve a high degree of conversion for all three of the aforementioned harmful substances in exhaust gases, the normalized air/fuel ratio needs to be set within a very narrow range around xcex=1 (stoichiometric condition). The interval around xcex=1 within which at least 80% of all three harmful substances are converted is often called the lambda window. To remain within the lambda window, the normalized air/fuel ratio may be adjusted with the aid of the signal from an oxygen sensor (lambda sensor). A two-point lambda sensor is usually used for this purpose. Due to the unavoidable inertia of the control system, this two-point regulation leads to modulation of the normalized air/fuel ratio with a frequency of about 1 Hz. Modulation of the normalized air/fuel ratio can be largely avoided by the use of a linear lambda sensor.
In order to treat exhaust gases from stoichiometrically operated internal combustion engines, one may use one or more catalysts. One type of catalyst that may be used is the three-way catalyst, which can simultaneously remove carbon monoxide, hydrocarbons and nitrogen oxides from the exhaust gas.
In order to prevent impairment of catalyst efficiency due to modulation of the normalized air/fuel ratio or by short-term variations in the normalized air/fuel ratio, modern three-way catalysts contain oxygen storage components (OSC), which store oxygen in the presence of a lean exhaust gas (xcex greater than 1) and release oxygen in the presence of a rich exhaust gas (xcex less than 1). Thus, they adjust the stoichiometry of the exhaust gas to xcex=1. Any compounds that permit a change in their oxidation state are suitable as oxygen storage components in a catalyst. Cerium oxide, which can be present either as Ce2O3 or as CeO2, is most frequently used.
In the present disclosure, the storage capacity of the oxygen storage component refers to the mass of oxygen that can be absorbed per gram of oxygen storage component. Accordingly, the momentary xe2x80x9cfilling degreexe2x80x9d is defined as the ratio of the mass of oxygen actually stored to the storage capacity. The storage capacity of an oxygen storage component can be determined experimentally by various processes that are known to persons skilled in the art.
By regulating the normalized air/fuel ratio, one is able to avoid complete filling or complete depletion of the oxygen storage component. In the case of complete filling of the catalyst with oxygen, breakthrough by lean exhaust gas takes place, and this leads to the emission of nitrogen oxides. In the case of complete depletion, breakthrough by rich exhaust gas takes place and this leads to emissions of carbon monoxide and hydrocarbons. According to U.S. Pat. No. 4,024,706, the lambda window for three-way catalysts with oxygen storage components can be enlarged by suitable modulation of the air/fuel ratio. U.S. Pat. No. 4,024,706 is incorporated by reference herein. The modulation amplitude of the air/fuel ratio is preferably chosen to be less than 1, and the modulation frequency is preferably chosen to be greater than 1 Hz. If the modulation frequency is too low, there is a risk that the storage capacity of the oxygen storage might be exceeded during the lean half cycle of modulation and lean exhaust gas then would break through the catalyst.
Regulating the normalized air/fuel ratio with a single lambda sensor upstream of the catalyst does not enable the stoichiometry of the exhaust gas to be adjusted with sufficiently high precision in order to avoid complete depletion or filling of the oxygen storage components over the long-term. For this reason, more recent motor vehicles are fitted with a second lambda sensor downstream of the catalyst that detects the breakthrough of rich or lean exhaust gas and counteracts it. This is called xe2x80x9clead control.xe2x80x9d However, the xe2x80x9clead controlxe2x80x9d system detects complete filling or depletion of the oxygen storage components only when the breakthrough of rich or lean exhaust gas has already taken place. Thus, depending on the reaction time of the control mechanism, a significant release of harmful substances is unavoidable.
According to DE 196 06 652 A1 (U.S. Pat. No. 5,901,552), which is incorporated by reference herein, the air/fuel ratio supplied to an internal combustion engine may be varied in such a way that the filling degree of the oxygen storage in a three-way catalyst is always located between an upper and a lower limit. To implement this control system, the oxygen content in the exhaust gas upstream and downstream of the catalyst is measured and the measured values are evaluated with the aid of a mathematical model. In this way, the catalyst can, at any time, detect an unexpected occurrence of lean or rich deviations in the exhaust gas and thus avoid the breakthrough of emissions. The disadvantage of this system is that, over a long operating period, an increasing difference between the actual filling degree and the theoretically determined filling degree can occur.
The object of the present invention is to provide a process for regulating the filling degree of the oxygen storage component within a predetermined set-point interval on the basis of measurements of the normalized air/fuel ratio of the exhaust gas upstream and downstream of the catalyst.
The present invention provides a process for operating a three-way catalyst that contains an oxygen storage component that has a minimum and maximum filling degree for oxygen and that is located in the exhaust gas line of an internal combustion engine. According to this process, the air/fuel mixture supplied to the engine is varied in such a way that the momentary filling degree of the oxygen-storage component in the catalyst is held within a set-point interval between the minimum and maximum filling degree.
According to the present invention, in order to regulate the air/fuel mixture, migration of the filling degree out of the set-point interval is checked in a test phase in such a way that the filling degree is increased or lowered relative to the instantaneous value (initial value) by short-term enrichment or reduction in richness of the air/fuel mixture supplied to the engine by a certain amount and immediately returning to the initial value by a short-term opposing change in the air/fuel mixture (lean/rich pulse sequence or rich/lean pulse sequence).
In the case of a breakthrough of lean or rich exhaust gas through the catalyst during the test phase, the air/fuel mixture is enriched or reduced in richness in order to correct the filling degree. The amount by which the filling degree during the test phase is increased or decreased is such that no breakthrough of lean or rich exhaust gas takes place through the catalyst when the filling degree of the oxygen storage component is within the set-point interval. Thus, the present invention provides a process for operating a three-way catalyst by regulating an air/fuel mixture supplied to an internal combustion engine, wherein said three-way catalyst comprises an oxygen storage component and said oxygen storage component comprises a minimum filling degree and a maximum filling degree for oxygen and said oxygen storage component further comprises a set-point interval, said process comprises:
a. initiating a test phase, wherein said test phase comprises applying a pulse sequence to change a filling degree of the oxygen component, wherein said filling degree has an initial value and said pulse sequence either enriches or reduces the richness of the air/fuel mixture supplied to the engine by a certain amount to raise or to lower said initial value and then returns the filling degree to said initial value by applying a pulse in the opposite direction; and
b. determining whether there was a breakthrough of lean or rich exhaust gas through the catalyst during said pulse sequence.